There is desired in the petroleum refining industry, a petroleum refining technique which is capable of producing a high-quality light petroleum oil in high yield from a low-quality heavy oil as the charge stock as one of the measures for more effectively utilizing the natural resources.
Nevertheless to the best of the present refining techniques, an attempt to enhance cracking efficiency results in deterioration of the petroleum product due to an increase in sulfur content and besides causes the trouble with petroleum refinery due to increasing sludge formation; while an attempt to suppress sulfur content leads to lowered cracking efficiency. Such being the case, there is not available a refining technique satisfying the above-mentioned requirements.
Under such circumstances, a cracking treatment technique which is capable of producing a product oil with a low sulfur content and a high cracking efficiency and is free from the formation of sludge is eagerly desired.
As the primary contributor to the trouble with petroleum refining at the time of a high degree of cracking reaction or caused by a rise in reaction temperature accompanying the deterioration of a catalyst, sludge formation in the refinery has heretofore greatly taken part therein, sometimes making the operation impossible.
A number of reports are made on the technique which suppresses the formation of sludge in the process directed to a high degree of cracking reaction. However, each of them is nothing but an abstract report failing to elucidate the working effect of the technique.
For example, Japanese Patent Application Laid-Open No. 25889/1990 discloses a two-stage process for hydrogenating a heavy petroleum oil by the use of a hydrogenation catalyst having a large pore diameter of 150 .ANG. or larger in average which comprises the first step in which the hydrogenation is carried out at a reaction temperature of 400.degree. C. or lower and the second step at a reaction temperature of 400.degree. to 460.degree. C. However, any of the working examples in the above-mentioned disclosure describes the reaction by means of an autoclave only, thus failing to elucidate the extent of decrease in the formation of sludge in a fixed-bed flow type reactor as a commercial production equipment. In addition, it never describes the quality of the hydrogenated product oil. Moreover, the above-disclosed process involves the problem that as compared with the previous direct hydrodesulfurization process, the two-stage hydrogenation process necessitates a heating furnace facility, thereby being accompanied by an increase in the process cost.
The technique disclosed in Japanese Patent Application Laid-Open No. 305891/1990 relates to a descaling agent which removes the suspended solids such as the scale contained in heavy hydrocarbon oil, a hydrogenation catalyst which removes the dissolved metals such as organometallic compounds in heavy hydrocarbon oil and the methods of using said agent and catalyst, respectively. As the specific method of using, the above-mentioned descaling agent is packed in the forefront of a reactor to prevent catalyst solidification or clogging by scale at the forefront of the reactor.
However, since the sludge that is the object of the present invention is formed on the downstream side of the reactor, the descaling agent is judged to be incapable of exerting the descaling effect.
Aside from the aforestated disclosures, the technique disclosed in Japanese Patent Application Laid-Open No. 243196/1988 relates to a process for hydrocracking a heavy oil as the charge stock by the use of a hydrogenation catalyst in the coexistence of a hydrogen-donative solvent and hydrogen gas, followed by the hydrogenation of the produced oil. According to the working examples of the disclosure, although the cracking efficiency is enhanced by the process, the sulfur content in the product oil is still high, thus deteriorating the quality of the product. It is naturally anticipated that the process running cost is raised by the use of the hydrogen-donative agent. Furthermore, the hydrogenation process does not show the effect on decrease in sludge formation, revealing itself to be not so valuable as being expected.